This invention embodies an improved membrane comprised of a microporous lyophobic layer having deposited thereon an essentially nonporous lyophilic coating.
Conventional distillation requires heating the distilland to the boiling point of one or more of the constituents of the distilland and then recovering and condensing the resulting vapor to a liquid, the distillate.
Distillation has also been accomplished at much lower temperatures through the use of a membrane across which a vapor pressure gradient is maintained. Previously, porous lyophobic membranes were used for thermal distillation and desalination.
The terms "lyophobic" and "lyophilic" are employed here to describe the wettability of a liquid on a solid surface. Lyophobic describes a non-wetting behavior of a liquid such that the contact angle between a liquid and a solid is greater than 90.degree. so that the liquid tends to ball up and easily run off the surface. Lyophilic describes a wetting behavior of a liquid such that the contact angle between a liquid and a solid is less than 90.degree. and the liquid easily spreads over the solid.
The contact angle or wettability depends on the relative molecular attraction between both the solid and liquid and the liquid molecules themselves. If the attraction of the liquid for the solid is more than that for itself, the contact angle is less than 90.degree.; wetting is said to occur and the liquid is said to be lyophilic with respect to the solid. A liquid is lyophobic with respect to a solid when the attraction of the liquid for the solid is less than that for itself thereby creating a contact angle greater than 90.degree..
When the liquid is water, the terms hydrophobic and hydrophilic are generally substituted for the more inclusive designations.
It is the primary object of this invention to provide an improved membrane and method for thermal distillation which prevents water-logging within the pores of the membrane.
Another object of this invention is to improve the rate of thermal distillation by providing a thinner membrane.
Another object of this invention is to prevent back-streaming of the distillate into the distilled and the concomitant loss of distilled product.
Another object of this invention is to provide a unitary composite membrane which is not subject to mechanical problems resulting in separation of the membrane layers.
The accomplishment of these objectives is explained more fully below.